1. Field of the Invention
The invention relates to an electric power steering system including a rack housing and a drainage device.
2. Description of the Related Art
Electric power steering systems (EPS) have a higher degree of flexibility in layout and consume smaller amount of energy than hydraulic power steering systems. Therefore, in recent years, adoption of electric power steering systems not only in small-sized vehicles but also in large-sized vehicles has been under contemplation. In response to this, a demand for high-powered electric power steering systems has been increasing. In order to meet such an increasing demand, so-called rack assist-type electric power steering systems have become widely available. In a rack assist-type electric power steering system, a rack shaft of a rack pinion-type steering mechanism is equipped with a ball screw mechanism capable of producing a high rack axial force with a high speed reduction ratio, and assist force from an electric motor is transmitted to the rack shaft via the ball screw mechanism. A known rack assist-type EPS is a belt-driven-type EPS that includes an electric motor disposed parallel to a rack shaft, a speed reducer in which and a belt is looped over a pulley provided on a shaft of the electric motor and a pulley provided on a ball nut rotatably fitted to a ball screw shaft portion of the rack shaft, and a rack housing that houses these components. The driving force produced by the electric motor is transmitted from the ball nut to the rack shaft via the belt. Because the EPS is attached to a lower part of the vehicle body, which is relatively close to a road face, end portions of the rack shaft are covered by resin boots that are formed in a bellows shape.
If gravel or the like hits the boot while a vehicle is travelling on a rough road and its impact force damages the boot, water may enter the rack housing from the damaged part. The water that has entered the rack housing is not naturally drained. If the vehicle is placed in a low-temperature environment with the water left inside the rack housing, the water remaining around the speed reducer may freeze, raising a possibility that the operation of the speed reducer will be impeded. In the belt-driven-type EPS, in particular, the belt and the pulley are stuck together so that the belt cannot be driven. For example, the specification of German Patent Application Publication No. 10 2009 039 832 describes an EPS in which a rack housing is equipped with a drainage device and thus water that has entered an internal space of the rack housing can be drained therefrom. As illustrated in FIG. 5, a drainage device 200 includes a drain case 210 formed of a first drain case 220 and a second drain case 230, and a drain valve 240 formed of an elastic member. The first drain case 220 has a case inlet 221 that communicates with an internal space of the rack housing, and the second drain case 230 has case outlets 231 that communicate with the outside of the rack housing. The drain valve 240 has a main body 241 and a flange 242. The flange 242 contacts a protrusion 222 formed on an inner face of the case inlet 221, thus closing the case inlet 221. The water that has entered the internal space of the rack housing flows into the drain case 220 through the case inlet 221, and accumulates on the flange 242. Then, due to the pressure of the water, the flange 242 elastically deforms as illustrated in FIG. 6, so that a gap is formed between the flange 242 and the protrusion 222 formed on the inner face of the case inlet 221, and the water flowing out through the gap is drained through the case outlets 231 to the outside of the rack housing.
However, in the drainage device 200 of the above-described EPS, the water uniformly spreads and accumulates on the flange 242 of the drain valve 240. This makes the water pressure low, resulting in low water drainage performance. Furthermore, because the area of contact between the flange 242 and the protrusion 222 formed on the inner face of the case inlet 221 is small, the sealing performance is low. Therefore, for example, when the drainage device 200 is submerged in water, the water may enter the internal space of the rack housing through the case outlets 231.